Source of nicotinamide governs its metabolic fate in cultured cells, mice, and humans

Metabolic routing of nicotinamide (NAM) to NAD⁺ or 1-methylnicotinamide (MeNAM) has impacts on human health and aging. NAM is imported by cells or liberated from NAD⁺. The fate of ²H₄-NAM in cultured cells, mice, and humans was determined by stable isotope tracing. ²H₄-NAM is an NAD⁺ precursor via the salvage pathway in cultured A549 cells and human PBMCs and in A549 cell xenografts and PBMCs from ²H₄-NAM-dosed mice and humans, respectively. ²H₄-NAM is a MeNAM precursor in A549 cell cultures and xenografts, but not isolated PBMCs. NAM released from NAD⁺ is a poor MeNAM precursor. Additional A549 cell tracer studies yielded further mechanistic insight. NAMPT activators promote NAD⁺ synthesis and consumption. Surprisingly, NAM liberated from NAD⁺ in NAMPT activator-treated A549 cells is also routed toward MeNAM production. Metabolic fate mapping of the dual NAM sources across the translational spectrum (cells, mice, humans) illuminates a key regulatory node governing NAD⁺ and MeNAM synthesis.